The present application relates to fluid jet print heads and, more particularly, to a stimulation arrangement of the type which produces pressure varicosities in the individual fluid jets, resulting in substantially uniform breakup of the jets into streams of drops.
Ink jet printers, incorporating fluid jet print heads, are known which have an orifice structure defining a plurality of orifices. The orifices receive an electrically conductive recording fluid, such as for example a water-base ink, from a pressurized fluid supply manifold and eject the fluid in one or more rows of parallel streams. As the streams break up into drops, the drops are selectively charged and deflected, with some of the drops being deposited on a print receiving medium and the balance of the drops being caught by an appropriate catcher structure.
Charging of the drops is accomplished by selectively applying charging voltages to charge electrodes positioned near each of the streams. The fluid flowing through each orifice emerges as a fluid filament. Drops break away from the tip of the fluid filament and carry charges related to the voltage of the associated charge electrode at the instant of drop formation. Each drop is then subjected to an electrostatic field which deflects the drop by a distance proportional to the magnitude of the charge which it carries. Drops may thus be deflected to one or more print positions and, when a drop is not to be deposited on the print receiving medium, deflected to an adjacent catcher structure.
With print heads of the type used in ink jet printers, it is necessary to control drop formation since if left to natural stimulating disturbances, the fluid filaments would break up erratically into drops of various sizes at irregular intervals. Such erratic drop formation would prevent proper charging and deflection of the drops. Accordingly, it is customary to apply a stimulating disturbance to all of the fluid streams to produce jets of uniformly sized and regularly spaced drops.
Various types of stimulation arrangements have been suggested. U.S. Pat. No. 3,739,393, issued June 12, 1973, to Lyon et al, discloses an ink jet print head in which the fluid orifices are defined by a thin, relatively flexible orifice plate. A piezoelectric transducer contacts the orifice plate at one end and produces a series of bending waves which travel longitudinally along the plate. Dampers at each end of the orifice plate dampen the traveling waves and prevent wave reflection. The bending waves in the orifice plate produce an oscillatory movement of the orifices which, in turn, causes pressure varicosities in the fluid filaments emerging from the orifices. As a consequence, the fluid filaments break up into relatively uniform jet drop streams.
It will be appreciated that break up of the drop streams is nonsynchronous in a print head employing traveling wave stimulation. The print head, therefore, cannot be operated at its maximum printing resolution since the precise time of drop formation for each stream will be unknown and charge voltages must be supplied to the charge electrodes for sufficient time periods to insure that they result in appropriate charging of at least one drop. As a consequence more than one drop is usually charged in succession and partially charged drops, formed during charge voltage transition periods, are commonly formed.
One solution to these problems is to apply drop stimulating disturbances to all filaments in synchronism. If all of the jets have the same diameter and velocity, and stimulating disturbances are applied to the jets simultaneously, all filaments will generate drops in synchronism. Such synchronized drop generation greatly simplifies the application of charge signals to the charge electrodes, because the timing for each of the jets is precisely the same. Additionally, charge voltage transitions can be timed to occur between drop formations. The number of partially charged drops is therefore substantially reduced. Providing such precise synchronized stimulation to all of the jet drop streams in a long row of streams is not a simple matter, however.
U.S. Pat. No. 4,095,232, issued June 13, 1978, to Cha, discloses a print head in which stimulation is provided by flexing a pressure plate mounted on the opposite side of the fluid manifold from the orifice plate. A plurality of piezoelectric transducers are positioned along the length of the pressure plate on the opposite side thereof from the manifold. The transducers are stimulated in unison so as to produce oscillation of the pressure plate which is in phase along its entire length. This approach requires a substantial amount of mounting structure for the transducers and, additionally, requires that all of the transducers operate in precise synchronization and at substantially the same amplitude. If one or more of the transducers operate slightly off frequency, or at a lower amplitude, it is possible that traveling waves may be produced which move along the pressure plate, causing nonsynchronous drop generation. Additionally, the stimulation amplitude may vary along the length of the print head, producing fluid filaments of differing lengths.
U.S. Pat. No. 4,138,687, issued Feb. 6, 1979, to Cha et al, discloses a print head having an elongated piston mounted in the upper portion of the fluid manifold. A number of piezoelectric transducers are mounted along the length of the piston to produce vertical movement thereof and stimulation of fluid jets. The piston has a plurality of transverse slits along its length which are alternately cut from opposite upper and lower surfaces. The slits are more than one-half of the height of the piston such that there are no horizontal planes through the piston which are not cut by at least some of the slits. These slits minimize wave propagation along the piston which would otherwise cause deterioration of the stimulation process.
It will be appreciated that prior art mounting structures for piezoelectric transducers used in a print head having a stimulation piston or pressure plate arrangement are relatively complicated and add substantially to the cost, size, and weight of the print head. It will be appreciated, also, that multiple transducer stimulators in the prior art have been subject to operating difficulties when the amplitudes of the vibrations produced by the transducers have not been substantially uniform.
Accordingly, it is seen that there is a need for a stimulation arrangement not having the limitations associated with prior art fluid jet stimulation devices.